1. Field of the Invention
This invention relates to the recovery of asymmetrical binary signals and specifically to the amplification of such signals from a very low level to a voltage and power level acceptable to digital circuitry.
2. Description of the Prior Art
In the past, it has been necessary to use DC coupled amplifiers to amplify pulse code modulation (PCM) nonreturn-to-zero (NRZ) signals because of the varying duty cycle of the pulses. The varying duty cycle results from the fact that the signal does not return to zero between consecutive ones. There is, however, an advantage in using this form of transmission because it conserves bandwidth.
AC coupled amplifiers can be made more sensitive than DC amplifiers because they can be designed with high gain, and stabilized with a DC feedback loop that does not reduce their gain to the AC component. The disadvantage of AC coupled amplifiers is that they exhibit what is known as DC shift when the signals they are amplifying are not symmetrical. That is, there will be a false DC component in the output of an AC coupled amplifier that is relative to the average level of the signal seen at the input. This would be unacceptable in a digital receiver in that it would cause errors in the decision making circuit (usually a voltage comparator) that interfaces the amplifier to the digital circuitry.
In view of the unacceptable false DC component which results from the use of an AC coupled amplifier, consideration has been given to the use of high gain DC amplifiers for PCM NRZ signals. Such amplifiers are, however, very difficult to stabilize because slight variations in bias currents, due to changes in temperature and component aging, are greatly amplified. Feedback cannot be used in a conventional DC coupled amplifier for stabilization without correspondingly reducing the gain to the desired DC component.
Bipolar transmission, and other schemes that condition the signal so that its duty cycle is 50% have been employed so that AC coupled amplifiers could be used. In a low power fiber optic system, however, bipolar transmission is not practical and other schemes require extra bandwidth, which places its own demands on the system power budget. Therefore, in designing such a low power fiber optic system, the PCM NRZ format was chosen for transmission of the information.
The amplifier described herein was designed for use in the low power fiber optic system described above. The amplifier has the high gain and stability of an AC coupled amplifier and yet can accommodate wide variations in duty cycle without suffering from DC shift. There is a limit to the maximum pulse width it can sustain before DC shift sets in, but that limit is adjustable. In the embodiment herein described, the limit is greater than one frame of the bit stream. Therefore, as long as there is at least one zero bit per frame all of the other bits in the frame can be ones without evidence of DC shift.